Matching market needs

As competition in the market for industrial power generation equipment rises, small gas turbine manufacturers are increasingly looking to increase both reliability and efficiency. GE Nuovo Pignone recently unveiled its latest machine aimed at this market, the first unit of which will be installed at an Italian paper mill.

Erio Benvenuti, Dino Bianchi,

GE Nuovo Pignone,

Florence, Italy

Paper production requires nearly constant generation of three types of energy: electricity, steam and hot air for paper drying. These conditions are ideal for cogeneration with a gas turbine, which can provide all of the heat at the high temperatures required in the paper mill processes. This maximizes the use of exhaust heat and reduces total energy costs – a key factor in the final product cost.

GE Nuovo Pignone of Florence, Italy, is a leading supplier of gas turbines for paper mill applications. Over the past several years, its PGT gas turbines have been installed for cogeneration projects at five paper mills in Italy, resulting in energy costs being reduced by an average of 30 per cent in each case. The company`s latest paper mill project involves its newest gas turbine, the PGT5B. It will be installed at Cartiera di Vignaletto`s paper manufacturing facility in Verona, Italy, as part of a plant expansion, which requires increased steam and electrical output.

A 2 MW PGT2 gas turbine from GE Nuovo Pignone already is supplying electric power and steam for one production line at the Cartiera di Vignaletto plant. Another line is being added which will require about 4700 kW of power, a steam supply of 7500 kg/h, and hot air for dryers. The PGT5B was selected for the new line because its power output meets the application requirement, and its efficiency is higher than other units of a similar size.

The new gas turbine will be installed in January of 2000, following the completion of factory testing. It will undergo an endurance test on site, which will include monitoring to verify performance and part life prediction. A long-term service agreement covers the 6000-hour test period with special guarantees, and will cover the normal operation of the new machine for six years. The contract also includes a remote monitoring and diagnostics system.

Rugged technology

Introduced at in June this year, the PGT5B combines the technology of GE aircraft engine designs with the ruggedness of the heavy duty PGT-class turbines. A 5.9 MW machine, it has a single-shaft configuration for power generation applications. A two-shaft version for mechanical drive applications is scheduled to be available in the second half of 2000. The PGT5B offers a net efficiency at generator terminals of 31.9 per cent and will restrict NOx emissions to 25 ppm on natural gas and 42 ppm on liquid fuel, meeting stringent environmental requirements.

Synergy between GE Aircraft Engines and GE Nuovo Pignone`s gas turbine division has led to this latest evolution in the PGT line of gas turbines. The new machine builds on the worldwide experience compiled by the PGT line, along with the millions of hours of testing and reliable operation accumulated by GE aircraft engines.

The PGT5B development programme led to a robust design incorporating dual fuel flexibility with a range of options for quick and easy on-site maintenance. The flexibility, simplicity and compactness of the new machine, along with its high reliability and availability, make it well suited for industrial power generation, oil and gas applications in remote areas, and offshore installations.

The PGT5B`s exhaust energy can provide a substantial quantity of steam at various pressures and temperatures. For applications that have a heating/cooling load, in hospitals, coupling it with a heat recovery steam generator can increase for example, the value and efficiency of the PGT5B. In this arrangement the exhaust energy, which otherwise would be wasted, is harnessed for steam production, saving the cost of extra fuel and an auxiliary boiler. This cogeneration of electricity and steam frequently enhances the overall economics, depending on the cost of electricity and gas the user currently is paying.

Industrial users such as Cartiera di Vignaletto, who need steam and electrical power also can benefit from the long maintenance cycles associated with the gas turbine system. Designed for high reliability and availability, the PGT5B will allow the industrial user to overcome lost production due to utility interruptions while planning gas turbine maintenance during normally scheduled plant shutdowns.

Design features

The PGT5B 11-stage axial compressor is a scaled down version of the PGT10B gas turbine. The new turbine introduces the following features:

•Advanced 3-D aero-design blading with the first five stages transonic, for 15:1 design pressure ratio.

•Optimal design characteristics for cogeneration applications, with possible turbine operation from 50 per cent output to nominal load at maximum exhaust temperature, implementing variable inlet guide vanes and stage one and two stators.

•Conveniently located bleed at the fourth stage for extraction of sealing air and a second bleed at the seventh stage for extraction of the cooling air required by the turbine hot section. Additional bleeding is included for the startup and shutdown transients.

•Solid forged rotor with assembled compressor blades, and stator blades assembled onto the horizontally split ductile cast iron casing.

The aerodynamic design of the compressor is exactly scaled from the PGT10B, which underwent extensive development work to achieve top performance while keeping the number of stages to a minimum. The initial design of the PGT10B compressor was intended to replace the original PGT10A, increasing the flow by about ten per cent to uprate the engine while also reducing production cost by substantially decreasing the number of stages and airfoils. This initial design, based on conventional aerodynamics, was tested on a special experimental setup, and demonstrated excellent mechanical performance and a wide operating range, with efficiency similar to that of the replaced compressor.

An airfoil redesign was later launched with the support of GE`s aero-engine fan and compressor design group, to take advantage of the latest advances in aerodynamic design technology based on 3-D computational fluid dynamics. Conventional airfoils were replaced by customized airfoils, each individually designed to optimize Mach number distributions and to minimize shock and profile losses. An outstanding feature of this advanced design technique is a special 3-D airfoil stacking method, which minimizes secondary flow effects and boundary layer flows that lead to local flow separations and increased losses.

The combination of custom airfoil shapes and the fully 3-D design led to total elimination of local airfoil flow separations, with a gain in efficiency of up to two per cent. Extensive tests have demonstrated that the efficiency remains consistently close to maximum for pressure rations between 15:1 and 17:1 accompanied by a further increase in the surge margin. These results clearly prove the suitability and success of the PGT10B compressor design, and of the PGT5B as a scaled down version.

Advanced cooling design features of the axial flow, two-stage, reaction type turbine section include:

•Effective film-type air cooling on the first stage stationary nozzles and rotor buckets, with added cooling of the nozzles by internal impingement.

•Sophisticated multi-pass serpentine equipped with turbulence promoters and trailing edge exit holes for internally convection-cooled buckets.

•Finely detailed `shower-head` distribution of cooling holes in the leading edge area of the airfoil for effective film cooling.

•Small nozzles to minimize shocks and pressure drops in the delivery of the cooling air through the rotating cavities.

All buckets are investment casings – RE80 for the first stage and RE108 for the second stage. All nozzle segments are constructed of FSX414; the first-stage nozzle ring is formed by 16 segments and the second stage by 14 segments. The air cooling system is designed to allow turbine inlet temperatures consistent with future uprates, with the introductory value set at 1200 degrees C.

One of the main objectives of the PGT5B turbine design was to keep maximum commonality between the single-shaft and the two-shaft models, both aerodynamically and mechanically. To achieve this goal, the two turbine stages of the single-shaft engine were placed in overhung positions, so converting to a two-shaft gas generator turbine simply requires removing the second stage and leaving all other components unchanged.

The PGT5B`s single annual combustor is made of Hastelloy X and features 18 dual-fuel type nozzles with pilot flame and a pre-mix system, and dry low-NOx capabilities. The system has the capability to alternate between liquid and gas fuel at any load. At turbine ignition (no load) the pilot flame accounts for 50 per cent of total fuel, a percentage that is progressively reduced to ten per cent at full load when 90 per cent of the total turbine load is used in the premix mode. On natural gas, the system achieves a NOx level of less than 25 ppmvd, and 42 ppmvd for liquid fuel operation from 50 to 100 per cent of the shaft power.

The rotor is supported at both ends by two tilting-pad type radial bearings, while the thrust bearing is located at the cold end of the shaft. The main gearbox is mounted onto the common baseplate and connected to the rotor shaft through a flexible coupling. The main function of this coaxial-epicyclic gear is to reduce the turbine rotor speed (16 630 r/min) to the speed required by the four-pole generator – 1500 or 1800 r/min for 50 or 60 Hz grids respectively.

Mounted on the gearbox casing are the main oil pump and the hydraulic starter, provided with an overunning clutch. The oil pump circulates the mineral oil contained in the tank located inside the baseplate, and serve both the turbine and gearbox lubricating and cooling requirements.

The PGT5B will be commonly supplied as a completely enclosed, sound-proof package with a standard attenuation level of 85 dB(A) at 1 m. This package includes the entire baseplate containing the gas turbine assembled with the main gear and various accessories. The generator is mounted directly on the concert foundation block outside the lagging. A two-stage filter and a silencer are located above the turbine. The intake air filtration system, consisting of a pre-filter followed by a high-efficiency design filter, is protected by louvres and can be provided with automatic de-icing system when the turbine is operated in low-temperature environments.

A carefully planned safety system includes a high-pressure, automatic CO2 fire fighting system designed to meet US National Fire Protection Association requirements. The control system serves the entire gas turbine-generator set and offers high functional flexibility through the use of different software programmes. Main functions of the control system are turbine sequence and protection, fault monitoring, fuel control, turbine speed control, turbine temperature monitoring, generator bearing and winding temperature monitoring, turbine vibration monitoring, and fire and gas monitoring.

The entire package has been designed for maximum accessibility, versatility and safety, to simply maintenance operations. Boroscope inspection of the turbine hot section is planned for 8000 hour intervals, while the visual inspection of the same components (with an open casing) is scheduled at 16 000-hour intervals. The main overhaul is scheduled after 48 000 hours.

The prototype test programme at GE Nuovo Pignone`s Florence, Italy facility was completed in June of this year. The turbine, coupled to its generator, underwent full-load tests, which showed optimum mechanical and aerodynamic performance. The outstanding results obtained from the extensive test programme have shown that the assumptions and design considerations adopted in the development of the PGT5B were conservative, suggesting a higher than expected level of reliability.

A growing family

The PGT5B is the latest extension of the PGT family of gas turbines. Over the years, machines have been used for mechanical drive applications in such industries as paper, petrochemical and oil and gas. Today, their use for industrial power generation and cogeneration is increasing in key markets around the world that are seeing a steady rise in industrial power generation and autoproduction.

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Figure 1. The PGT5B is a 5.9 MW machine now available in single shaft configuration for power generation

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Figure 2. The turbine features aero-design blading

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Figure 3. Customized airfoils replace conventional airfoils

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Figure 4. An airfoil redesign was launched to take advantage of advances in 3-D computational fluid dynamics

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Figure 5. Improved blading optimizes Mach number distributions and minimizes shock and profile losses